1. Technical Field
The present invention relates to a method and a circuit for testing a high-frequency sound reproducing loudspeaker being part of a loudspeaker system.
2. Description of the Related Art
The output stages of loudspeaker systems, which are installed for instance on board motor vehicles, usually feature either a low frequency sound reproducing loudspeaker and a medium-frequency sound reproducing loudspeaker or a single medium-low sound frequency reproducing loudspeaker, which are generally directly connected to the amplifiers of such output stages.
An additional loudspeaker is usually provided, for reproducing high audio frequencies (also referred to hereinafter as “tweeter”), which is connected to the amplifiers of such output stages via a capacitor, as well as to the other loudspeakers.
Particularly, the operation of such loudspeaker systems is checked when they are installed in the vehicle.
Prior art diagnostic methods and circuits are known to be able to only ascertain the connect/disconnect state of the low and/or mid-frequency sound reproducing loudspeaker, because such loudspeaker is directly connected to the outputs of the output stage amplifiers.
A tweeter connected to the output stages via a capacitor cannot be tested using the methods and circuits developed for low and/or mid-frequency sound loudspeakers.
In view of obviating such drawbacks, it is known to use a circuit that implements a test during which an AC signal (typically an ultrasonic sine wave, e.g. at a frequency above 20 KHz) is transmitted to the tweeter and the current flowing in the tweeter is checked for its amplitude, to determine whether the tweeter is connected.
In recent times, Class D switching amplifiers are being increasingly used, also in the automotive field, and provide a much greater efficiency than Class AB amplifiers.
With reference to FIG. 1, there is shown a possible configuration of a bridge-type Class D switching amplifier 1 installed in a motor vehicle, which can drive a loudspeaker system 1A.
The bridge-type switching amplifier 1 is schematically composed of a left arm 2 and a right arm 3, each being coupled to a terminal of the loudspeaker system 1A via pass-band filters 5 and 6.
The left arm 2 has a first input 2A, a second input 2A′ and an output 2C, the latter being in feedback relationship with the second input via a feedback line 2B, and the right arm 3 also has a first input 3A, a second input 3A′ and an output 3C, the latter being in feedback relationship with said second input 3A′ via a feedback line 3B.
As shown in FIG. 1, each of the left arm 2 and the right arm 3 has a feedback arrangement thanks to a feedback line 2B and 3B at a point 2C and 3C of the circuit 1, upstream from the low-pass filter 5, 6.
The loudspeaker system 1A is embodied by a load 4, as shown in FIG. 2, which can consist, for example, of a combination of a low frequency loudspeaker 4A (woofer) and a high-frequency loudspeaker 4B (tweeter).
As is shown, the tweeter 4B is coupled to the woofer 4A via a filter 4C which can filter the high frequencies of the signal delivered by the amplifier 1.
Each of the low-pass filters 5 and 6 includes an inductor L1, L2 in series with a capacitor C1, C2.
Particularly, the inductor L1 is connected on one side to the output 2C of the left arm 2 of the amplifier, which output also acts as a virtual ground, and on the other side to the capacitor C1 and to a terminal 4D of the load 4; the capacitor C1 in turn having a terminal connected to the ground.
The same applies to the low-pass filter 6, in which the inductor L2 is connected on one side to the output 3C of the right arm 3 of the amplifier, which output also acts as a virtual ground, and on the other side to the capacitor C2 and to a terminal 4E of the load 4; the capacitor C2 in turn having a terminal connected to the ground.
During operation of the amplifier 1, the voltage at the output terminals 2C and 3C is a modulated square wave which is low-pass filtered by the filters 5 and 6 before being transmitted to the load 4, so that the audio component to be reproduced by the load can be extracted from the square wave signal.
If low-pass filtering were not provided, there might be electromagnetic compatibility problems (electromagnetic interference, EMI) and an unnecessary high power would be dissipated, thereby causing damages to the load.
In order to determine whether the tweeter 4D is actually connected to the terminals 4D and 4E, also with reference to FIG. 1, an electronic current-reading device 7 is provided, allowing measurement of the amplitude of the current Iload circulating in the tweeter 4B.
In this configuration, the test for determining whether the tweeter 4D of the loudspeaker system 1A is actually connected to the terminals 4D and 4E, according to a specific method, is performed by applying a test voltage VinAC varying in frequency, e.g. at a frequency above 20 KHz, to each input terminal 2A and 3A of the arms 2 and 3 of the amplifier.
Particularly, a voltage +VinAC may be applied to the input 2A, which voltage is replicated (at least ideally) by the feedback 2B, to the terminal 4D of the load 4, and a voltage −VinAC may be applied to the input 3A, i.e. a voltage opposite in phase to the voltage applied to the input 2A, which is replicated (at least ideally) by the feedback 3B to the terminal 4E of the load 4.
Nevertheless, the presence of the low-pass filters 5 and 6 causes problems in reading the proper current in the load 4: the low-pass filters 5 and 6 at the frequencies of the variable test signal ±VinAC, of about 20 KHz, do not correspond to an infinite load, but a current Ioutamp flows in such load 4, and adds to the load current Iload.
Thus, the current detection device 7 detects both the Iload current flowing into the load 4 and the current circulating in the capacitor C2 (or the capacitor C1 if the detection device 7 is coupled to the left arm 2 of the amplifier 1).
This may affect accuracy or make the method as described above for detecting the load 4 totally ineffective.
Also, with further reference to FIGS. 3 and 4, there are shown the results of two simulations of the circuit as shown in FIG. 1, in which the x axis indicates time in msec, and the y axis indicates current in amperes, when the load 4 is simulated as an impedance having a resistance value of 4Ω (see FIG. 4).
In both simulations, L1 and L2 are assumed to be 20 μH and C1, C2 are assumed to be 2 μF and Vout=4Vpeak (i.e. the potential difference between the points 4D and 4E when a sinusoidal peak voltage of +2V/−2V is applied to the input terminals 2A and 3A respectively).
Particularly, it can be noted that both the load current Iload and the current Ioutamp flowing through the low-pass filter 6 into the left arm 3 flow into the load 4, because the frequencies at which the variable test signal −Vin is applied do not correspond to an infinite load.
It should be noted that, for clarity, the simulations of FIGS. 3 and 4 do not account for the current associated with the output square wave, typically of a relatively low value, and reduced to a negligible value by other techniques, which are well known to those of ordinary skill in the art and will not be described herein.
Still with reference to such FIGS. 3 and 4, the results of such simulations show that the current Iload that flows into the load 4 and the current Ioutamp that flows in the right arm 3 can assume the following values:                if the load 4 is simulated by a 10 KΩ resistance (see FIG. 3), corresponding to a situation in which such load 4 is an open circuit, the current Ioutamp is in a range of peak values from −2A to +2A, whereas the current Iload that flows into the load is substantially zero;        if the load 4 is simulated by a 4Ω resistance (see FIG. 4), corresponding to a situation in which such load 4 is a normal load (i.e. a normal loudspeaker combination), the current Ioutamp is in a range of peak current values from about −1A to +1A, whereas the current Iload that flows into the load 4 is also in a range of peak current values from about −1A to +1A.        
Apparently, no accurate detection is possible if the load 4 is simulated by a 10 KΩ resistance (see FIG. 3) because, while the load current Iload has a negligible or zero value, the current Ioutamp is very high, of about 2A, due to the current that flows in the output filter 5.
In other words, the device 7 reads a current value that cannot be used to determine whether the load 4 is actually disconnected.